At the height of Superstorm Sandy, half of Metro-North Railroad's Hudson River line was underwater.

But by the end of the century, many parts of the commuter railroad could be underwater — every day.

If sea levels rise by 2.5 feet — less than half of the current worst-case projections — low-lying portions of the railroad that carries an average of 1,800 riders south from Poughkeepsie every weekday will be inundated at high tide.

Those projections have forced the Metropolitan Transportation Authority, which operates Metro-North, as well as Long Island Rail Road and New York City's subway and bus systems, to factor future sea-level projections into its five-year capital plans.

And it has added new responsibilities to old job descriptions.

"For Metro-North, I have become the guru of climate change, storm surge and sea-level rise," said Tobey Ritz, chief engineer of capital engineering at Metro-North.

Climate change and rising sea levels are part of the national consciousness today, Earth Day, which launched the modern environmental movement 44 years ago and has engaged millions of Americans from all walks of life.

Less than two months after Sandy hit, the MTA amended its 2010-14 capital plan to add $5.8 billion to protect assets against future severe weather events and enhance overall system resiliency.

More than $5 billion of that was targeted for the city's transit system, primarily its subways.

Metro-North was allocated $128 million to address vulnerabilities and make infrastructure enhancements.

A key project has called for the railroad to bypass two points that are prone to flooding — the Mott Haven Junction, where the Hudson, Harlem and New Haven lines merge in the Bronx and the Harlem River Lift Bridge.

When these points fail, Metro-North — the nation's largest commuter railroad with 280,000 daily passengers — is completely cut off from Manhattan.

"They have to (start planning)," Pleasant Valley resident Russ Bock said. "You can't make changes instantaneously. And if it takes years to do the planning, it's going to take years for them to implement whatever they decide upon."

But herein lies the challenge. Uncertainty remains over how much — and more importantly how fast — sea-levels will rise.

Tidal gauges in New York Harbor show sea level has increased about 15 inches in the last 150 years, according a report from the state Legislature's Sea-Level Rise Task Force. And the pace of change is increasing.

And yet, one estimate by the National Oceanic and Atmospheric Administration predicts with high certainty that by the end of the century, sea-levels will rise as little as 8 inches, or as much as 6.6 feet.

In other words, scientists are confident the Hudson River's shoreline in the coming decades will look different at high tide. But how much different remains to be seen.

This makes planning a challenge, particularly when weighing cost against benefit.

"I would say yes to that," said Ritz, the Metro-North chief engineer. "It's not so much for us to pick which study is right, but to look at the entire range, look at the time frames that are predicted and then consider when is the right time to act."

Over the last two years, Metro-North surveyed the entire Hudson line to create a track and asset inundation map that catalogs, mile by mile, the average elevation of any given location on the Hudson line.

The railroad then examined newly revised federal flood data and factored in sea-level rise projections that fall in the "upper mid-range," Ritz said.

From that, the railroad created a design standard 4 feet above the revised flood data.

"With every decision about investment and infrastructure, we now have a new criteria," MTA spokeswoman Marjorie Anders said.

The railroad has to balance the cost of future enhancements against the changing predictions, as well as the useful life of the system's elements.

"They are looking at the two-faced coin of over-investing too soon and the opposite, which is you under-invest and then you get clobbered," said Sacha Spector, director of conservation science at the Poughkeepsie-based environmental nonprofit Scenic Hudson.

An added challenge, Spector said, is the potential for a slowly developing issue like sea-level rise to evaporate from the collective consciousness.

Such creeping issues "are inherently difficult to deal with in our political structure," Spector said. "It's going to take some real leadership and real will to do something."

This year marks the end of the current five-year capital plan. Ritz said the railroad is looking at projects that will be in the next five-year program to make sure it considers future changes.

Spector said that a key piece of the puzzle is something far beyond Metro-North's control — that is, how much effort is put into limiting emissions of greenhouse gases scientists say are the driving factor behind climate change and sea-level rise.

"We have our hands on the knob of that stove top," Spector said. "We can't forget that we have the chance to turn it off, or turn it down."